Remove AppContext usage

[mbien]

JDK 27 removed the (webstart) AppContext mechanism, NB appears to rely on it in some places

e.g see #9522 + openjdk/jdk#29855 (many others)

[neilcsmith-net]

Note from Slack conversation - one of these usages is registering NetBeans' editor kits for use with JEditorPane::setContentType. This may not be feasible in future. This is currently done by registering a HackMap subclass of HashTable in the AppContext that can lazily lookup editor kits. Using the supported registration APIs to eagerly register might need to be done within each providing module, or by listening to module enablement, to handle module loading, ergonomics, etc.

Assuming we don't do further hacks, we might want to note that registration of NetBeans' editor kits / expectation is deprecated in the notes for NB31?